Control apparatus



P 1944- s. G. ISSERS'ILEDT 2,357,897

CONTROL APPARATUS Filed Dec. 18, 1941 66 INVENTOR.

' Smgfma ct. Issars1-&+

Patented Sept. 12, 1944 2,357,897 con'raor. APPARATUS Siegfried G.Isserstedt, Minneapolis, Minn assignor to Minneapolis-HoneywellRcxulator Company, Minneapolis, Minn., a corporation of DelawareApplication December 18,1941, Serial No. 423,498

17 Claims.

This invention relates broadly to an automatic control system for afluid fuel burner, and is more particularly directed to thattype ofapparatus employing a timing means for timing the various functions whenthe apparatus starts on an original call for heat and is shut down as aresult of the demand for heat having been satisfied? or as a result pf aflame failure at the burner.

The main purpose of this invention wasto design an automatic controlapparatus in which a very sensitive and fast acting combustion re-, sponsive device may be used. Where such a combustion device is used it isnecessary to provide some means for delaying the restart of the burnermotor following a shut-down, in order to provide for a proper scavengerperio Anotherobject of the invention is to provide athermal timing meansfor timing thevarious functions referred to above. 1

A still further object of the invention is to provide a timing meanswhich has a. starting position and a running position, and in which thetimer must be energized in one direction to cause it'to move from itsstarting to its running position and then must be energized in theopposite direction to cause it to move from its running position toitsstarting position. The invention contemplates the use of a thermaltiming means comprising a pair of thermal elements in which one of theseelements is energized to move the timer from its starting to its runningposition and in which the other of the-elements is energized to move thetimer from its running to itsStarting position.

It is an object of this invention, not only to time the scavenger'periodfor the burner by means of this thermal timer but also to delay thedeenergization'of the ignition device until a predetermined period oftime after the establishment of combustion. In addition, the presentinvention contemplates utilizing this same thermal timer to operate asafety switch for the purpose of locking the apparatus out in the eventthat combustion is not established.

These and other objects will readily become apparent as the followingspecification is read in the light of the accompanying drawing whichdiagrammatically illustrates a control system embodying the variousfeatures of my invention.

My automatic control apparatus may obviously be placed under the controlof many various types of condition responsive devices. For the purposesof illustration a'room thermostat has been indicated generally at Hi.'This room ther- I grammatically at H.

ment which is fixed at one end and which I carries at its free end aresilient contact blade I! and a second contact blade l3 which cooperate5 cause the contact blade I 3 ary contact i5. I

A combustio moved'to the position shown in the absence of combustion atthe burner and upon't'he establishment of combustion to move in acounter;

clockwise direction. It is intended that this arm shall be moved uponthe establishment or termination of combustion and therefore is not inany sense of the word a calibrated'device. 'The free end of the arm I8is connected to a switch arm again move the spring 20 overcenter withrespect to the contact arm [9 which will then be snapped from engagementwith its hot contact 22 into engagement withits cold contact 2i.

Indicated at 25 is a double helix thermostat. the F. A. Parsons Patent2,121,259. This thermostat is composed of a relatively long strip ofbimetal which is wound into the form of a helix and then that helix isagain wound in the form of a larger helix to form in eiiect' a doublehelix.

The total length of the bimetal is extremely large n with respect to itscross-sectional area and there'- fore th-is type of thermostat lendsitself admira bly to a system in which it is desired to/ heat a thermalelement by passing current directly 7 through the elementitself.

/ mostat is shown as comprising a bimetallic ele- I! The lower end .ofthis thermal element is at 30 upon the opposite end of the pivoted arm21. The opposite ends of the thermostats 25 and 29 are connected to-theopposite ends of a horirespectively with the stationary contacts and i5.The contacts II and I5 are so related to the, two contact blades I2 andI3, that on a decrease in temperature the bimetallic element ll, willcontract and first cause the resilient c'ontact;.

blade I! to engage its stationary contact and then on a further decreasein temperatureQwill to engage its station-.-

responsive device is shown diaw This device is shown as. comprising anarm I8 which is adapted to be type bimetallic For details of thisthermostat, see;

2:1 I I zohtal armll which in carries an insulating member 32 which isguided as shown at and; A coiled spring 35 loosely surrounds theinsulating. member 32 between the fixed guide member 33 andthe arm 3|for a purpose which will be apparent later as this description proceeds.comprisesia pair oi' resilient contact blades and 39: which are biasedto open circuit position butwhich are-held in circuit closed position-bymeans Ofqthe insulatingmember 82' as shown in;

If the insulating. meinberll, is raised; the out-turned end of theresilient; blade- 7 ssgwill dropoi!v of shoulder on the insulatingmemberslfand this bladewill be released tor;

movement-to open circuit position; The shoulderv lilz will: then be,latched on; the end-clothe blade thev d aw and theswitch 3U, 39 willbeeflectively locked in open position untilit is manually reset by;

means; of the reset button; It will be noted that-the long leg A2 01'the reset button- It will Indicated at 31 1s a safety switch 'whichholdfthe switch blade am open circuit position" while theshorter' leg onthe' s'witch button llis,

, movingthe blade 39 tol-theleit in ordertore'g.

leasethe insulating member 32. a This provides for atrip'free reset, orini other wordsIpreVents the switch-sans, from being manuallyheld mmclosed position. It will: be impossible to: the=switch I8, I! as long.as" the insulatingmem;

' m seum-poem. I

me hj s pivoted. as. s bwm at: .6. and carri v switch, Contact. m

41; and which cooperate with various sta'-.

contacts as in, the drawing. Switch arm 45 is connected'to extension 'llonxgthe pivoted arm I'l by means ,0! the over-g lishment oi -acircuit-i'rom'onesideotthesecen ary winding" through; conductor: It;safety switch blades .10 andltconductorls, :contaotlfl-g switch bladesl2 and II. t ll,- conductorycenter spring II so thatas'j -long as thepivoted: 7

time 1 1; is in its mid-position as shown inthe drawingithe switch. arm48 remain in 1 either oi;-'itsactuated poistions; Ifthearm 21isromeanclockwise directlon. the springii'is carrledikovercenter withrespect to the switch.

mums therefore this am is snapped-to its l opposite position as showndndotted lines on the drawing In; order tosnapthisjarm backto thepositionshownon the drawing, the pivoted-arm- ..'21 rnust move in acounter-clockwise direction t the opposite side 01' l its amid-positiom, thus carrying the overcenter spring 5| overcenter withrespectto the switch arm II.

o a My automatic control system also includes an electromagnetic relaywhich is diagrammatically shown as comprising a relay coil or winding SIand an; armature indicated by the dotted line 54. The armature 54 isadapted to-actuate therelay switch arms 55,:58, 51;" and 59. When therelay coil 53 is deenergizedthe armature 54 is in its-out positionasshown in the drawing, but whentheqrelay coil 53 is energized thearmature S'Lisinf its in position and the various relay switch arms arein engagement respectively with their stationary contacts 80; BI, 82,Bland 64.

Myv control system may be adapted to control many various types ofdevices for delivering fuel tea-burner, but for the purposes ofillustration the usual oil burner motor has been illustrated. Also,theusual electrical ignition device 61 for igniting the oil is shown.

Electrical power for my control system is suppliedv from a pair of linewires 88 and 10 which, through the conductors H and I2, supply powertothe primarywinding 13 of a step-down transformer l4. The transformerIt is provided with a pair of secondary windings indicated at and I6,-

means oi the insulating member 32;:

Operation Inthe drawing, the room thermostat is shown a in its satisfiedposition inwhich the contact .blades l2 and I! 'are out ofengagemen-twith their stationary contact I4 and iii Therefore; there isno current flowing through the winding "of the relay and the armature),is inits;out 1 position The-burner motor and ignition devicevarethereiore deenergized and there is no combastion-present at theburner with, the resultthatr the switch arm is has been actuated by thecom bustion responsive arm: ll intoengagement' with its cold contact21'. There is no current flowing through, the two, thermostats, IS -"and29.. andj'the'reforethefy are both at-the same temperature; which isroom temperature. The switch arm-1!! is shown inits right-handpositionsvThe safety" switch flJ-is :also being held in-v closed position by:-

As a result of the abovecondition thetemperae to cause; the; switchblade .ii to engage-iits tionary: contact I. This I results in:theiestab.

80, relay winding l3, conductors ll: and-82 swltchw I plate 41, andconductors, ",JMand-Niback to" t,

the other side of the secondary winding-II};

- circuit energizesthc relay winding-llwhich irn-1 v mediately attractsthe armaturejlandmausea the relay arms II: to iltoimove to theirclosedcircuitposition.

n agement of relay armiii with its stationary contact ".sets up aholding circuitirorn 'om side .of the secondary winding "lithroughjcon-rductor I8, safety switch blades '38 andgfl; con ductor I9, contact 14,switch blade 12; bimetallic element ll, conductor 88-, contact; relayzannn i, conductor 81, relay-winding", conductor's'tl -t and 8!, switchplate 41 and conductors"=83,'q1

and back to the other side ot'the secondary winding l5. This circuitholds the relay winding 53in, independently of theengagementotthe switchblade l3 andcontact ll, which very sel-c dom make with a very goodcontact pressure.

Engagement-oi relay'arm {9 with itsstatiofn'ary} contact establishes acircuit from the: supply line 10 through conductor 89, contact -64,relay arm 59, conductors-90 and Si, burner motor 66,- c and conductor 92back to the'othersu'pply line; A circuit through the ignitiondevice 61extends from the junction of conductors 90 and 91 v through conductor93, switch plate 4 8;,oond-uctorv 94, ignition device 61, andconductor"back-to the supply line 69. The "above two circuits: result in adelivery of fuel to the burner and -'the -op-- eration of the ignitiondevice 61'.

Engagement of'the relay arm 51 withrits stav tionary contact 62,establishes a circuit fromone, side of the secondary winding 16through'cone ductor 96, horizontal arm 3!, thermostat 25, con ductors 91and 98, relay arm 51, contact iconductors 99 and I00, cold contact2l,'switch-arm..

ll, and conductors HH and 84 back side -'of the secondary winding 16. a,Engagement'of relay arm 58 with its stationary contact 63, establishes acircuit from one side oi the 'seconda'ry winding 16 through conductor96,

to-the other horizontal arm 3|, thermostat; conductor I02;

contact 53;reli1y arm 58,1conductot )3, switch plate 41,and conductors83 and. back to the other side of the secondary winding 16. v

' The last two circuits traced pass through the thermostats 25 and'29and due tothe resistance of thefbimetal; generate heat therein, causingthese two thermostats to. expand. Inasmuch as the thermostats 25 and 29remain closed, thev insulating element 32 will be raised far enough topermitthe out-turned end of the switch blade .39'todrop oilthe'sh'oulder 40 andmove to open circuit position. The opening of thesafety, switch 38-, 39results in the deenergization of'therelay'win'ding 53 and hence the deenergization ofthe burner motor 65 andignition device 61.

and :will remain so until it'is manually reset by means of the resetbutton 4 I.

The above sequence of operation takes place in the event combustion isnot established as a result of the energization of the burner'motor."and ignition device 61. Normally, combustion will be established beforesufllcient time has elapsed to open the safety switch 31. soon .ascombustion is established, the combustion responsive arm l8 will snapthe switch arm l9 from engagement with its cold contact 2| intoengagement'with' its hot contact; The separation of the switcharm I9from its cold contact 24 breaks the circuit through the thermostat 25,and therefore this thermostat stops heating and begins to cool, but thecircuit through the thermostat-29 'rernains'closed and thereforethisthermostat continues to expand.

Engagement of the switcharm IS with its hot contact 22 establishes aholding circuit for the relay coil53'which extendsfrom one side of thesecondary'winding 15 through conductor 18, safety. switch blade 38"a1'1d39, conductor 19; con,- tact l4, switch blade l2,- bimetallic elementll, conductor 86,"contact 60, relay arm 55, conductor 81, relaywinding53, conductor8l', conductor I; contact S'Lrelay arm 56; conductorI06, hot

contact 22, switch arm 19, and conductors Nil.

84 and 85 back to theother side of the secondary winding 75. It willbe.noted that this holding circuit is independent of the engagement of thecontact plate 45 with its stationarycontacts.

It was noted above that upon the establishmentof combustion thethermostat 29 was expanding with'respect to the thermostat 25. Thisresults in a rotation of pivoted arm 21 in a clockwise direction' whichwill carry the spring 5! overcenter with relation to the contact arm 45.which will result in this contact arm being snapped from thepositionshown in solid lines to the position shown in dotted lines in thedrawing. This action b eaks the original starting circuit to the relaywinding 53 at 41, but this relay is held in by means of, the holdingcircuit traced The system is therefore eiiectively locked out above. Italso breaks the circuit to the thermostat 29 and thisthermostatfltherefore returns to the same temperature as the. thermostat25l In addition, it causes the'contact plate 49fto engage its stationarycontacts but this doesnot establish a circuit at the present time. Italso causes the contact plate 48 to separ'atefrom its stationarycontacts which results in the de energization of the'ignition device 61.This is the running condition of the circuit. v I

It: will be noted at this time that the relay winding was. energizedby'a circuit passing through the not contact 22 of the combustionresponsiveswitch. Therefore, upon] a failure oi combustion at the burner theswitch arm 19 will be caused to disengage the hot'contact- 22, whichwill result in deenergization of the relay winding 53. This drops outthe armature 54 and deen'er gizes the burner motor. 66. It will benoted-that the relay winding 53'c'annot again be energized untilthejswitch arm ret urn's'toj its solid line position causing thecontactplate 41 to engage its stationary contact. I g 3 When the combustionresponsive arm 18 moves the switch arm IS into engagement with thecoldcontact'Zl, it'establishes a circuit ,from one side of the secondarywinding 16 through conductor 96, horizontal "arm 3!,- thermostat-25,'conductors 91 and I08, contact plate 49; conductors I09 and I00,cold contact 2|, contact arm l9 and conduc: tors HH' and 84 back to theother sideof the secondary winding 16. This'circuit causes heating ofthe thermostat 25 which will expand and cause the arm 21 to rotate in acounter clockwise direc tion which will result in the mo'vement'ofv thespring 5! overcenter with respect to the contact arm 45 which will thensnap to its solid line position. The system will then recycle. Thetimerequired for the systemto recycle is therefore determined by the timenecessary forthe thermo: stat 25 to heat suiiiciently to returnthe-=contact arm 45 to its solid line position. g

If the system is in running condition and the room thermostatbecomessatisfied, the bimetallic element II will expand and cause the resilientcontact blade I2 to separate from the stationary contact l4. This willdeenergize the relay winding 53 which will in turn drop out the armature54 and deenergize the burner motor 66. This in turn willterminatecombustion at the burner and the combustion responsive arm l8 will snapthe contact arm l9 out of engagement with the hot contact 22 and intoengagement with the cold contact 2|. This will establish the circuitthrough the thermostat 25.'as traced above. which will return thecontact arm 45 to its solid'line position.

.As many changes and modifications will doubtless occur-to those who areskilled in the art, I

wish itto be understood that I intend to be limited by the scope-of theappended claims and not by the specific embodiment of my invention whichhas been disclosed herein for the purposes of illustration only.

I claimas my invention: 1 1. A fluid fuel burner control systemcompris-' ing in combination. means fordelivering fuel to a burnertimlngmeans, ajstarting switchea start,.

ing circuit including said starting switch for rendering said fueldelivery means o erative. circuit control means closin said c rcu t inres nse to a demand for burner operation, means. includin said timingmeans. operative a a re ult of the-closure of said starting circu t fors tting up a holding circuit for said fuel delivery means and openingsaid starting-switch, said'circuit control means opening said holdingcircuit and rendering said fuel delivery means inoperative when thedemand for burner operation is satisfied, and means operative as aresult of the deenergization sponse to a demand for burner operation,means,

including. said timing means, operative as a result of'theclosure ofsaid starting circuit for setting up a holding circuit for said fueldelivery means andopening said starting'switch and also rendering saidignition means inoperative, said circuit control means opening saidholding circuit and rendering said, fuel delivery means inoperative whenthe demand for burner operation is satisfied, andmeans operative as aresult of the deenergization of said holding circuit for energizing saidtiming means in amanner to cause it to reclose said starting switch.

3. A fluid. fuel-burner controlsystem comprising in combination, meansfor delivering fuel to a burner, timing means, a starting switch, astarting circuit including said starting'switch for rendering saidfueldelivery means operative, circuit control means closing said circuit inresponse to a demand. for burner operation, means operating in response.to the establishment of combustion to set up a holding circuit for saidfuel delivery means and energize said timing-means in a manner to opensaid starting switch, either said circuit-control means or saidcombustion responsive means being operative to open said holdingcircuit, and. means operative as a result of the opening of said holdingcircuit for energizing said timing means in a manner to reclose saidstarting switch 1 4. A fluid fuel burner control system comprising in,combination, means for delivering fuel to a burner, means for ignitingthe fuel, timing means, a starting switch, a starting circuit includingsaid switch for renderingsald fuel delivery. means and ignition meansoperative, circuit control means closing said circuit in response to ademand for burner operation,-means operating in response tothe-establishment of combustion to setup a holding circuitfor said ;fueldelivery means and energize said timing means in'a manner to open saidstarting switch, render said ignition means inoperative, either saidcircuit control means or said; combustion responsive meansbeingoperative to open said holding .circuit, and means operative as aresult of the opening of said holding circuit for energizing saidwtiming means in a manner to reclose said starting switch.

5. A fiuid fuel burner control system comprising in combination, meansfor delivering fuel to a burner, a timer member, a timing means whichmay be energized in a first direction to move said member from a firstto a second position and may be energized in a second direction to movesaid memberfrom said second to said first position, said timing meansbeing inoperative to move said member when it is energized in both orneither directions, a starting switch closed by said timer member whenit is in its first position, a starting circuit including said startingswtich for rendering said fuel delivery means operative, meansopisserative .as a result of the establishment of'combustion to close aholding circult=-forsaid fuel delivery means and energize saidtimin'gmeans in said first direction whereby said timer member is movedto saidsecond position resulting-in the opening of said starting switch,a second switch closed by said timer member in said second position, andcircuit means operative upon the opening of said holding circuit toenergize said timing means in said second direction tocause "said memberfrom said second to said first-position,

said timing means being inoperative tomove said member when it isenergized in both or neither directions, a starting switch closed bysaid timer member when it is in its first position, 'a'starting circuitincludingsaid starting switch for rendering said fuel deliverymeans-operative, means operative as a result of the establishment'of'combustion to close-a holding circuit forsaid fuel delivery means andenergizing said timing means in said first direction whereby said timermember is moved to said second positionresulting in the opening ofsaid-startin'g'switch, a sec'ondswltch closed by said timer member insaidsecond position, circuit means operative upon the openingof saidholding circuit to energ ze said timing means in said second directionto cause said timer member to reclose said starting switch, said circuitmeans including said second switch, and switch means operated by saidtimer member to dee ergize said timing means as said timer member movesfrom its first to its second position and from its second to its firstposition. j "I. A fluid fuel burner control system comprising incombination, means for delivering fuel to a burner, means for ignitingsaid fuel, a timer member, a timing means which may be energized in afirst direction to move said member from a first to'a second positionand may be energized in a second direction to move said member from saidsecond to said first position, said timing means being inoperativeto-move said member when it is energized in both or neither directions.a starting switch closed by said timer member when it is in its firstposition, a starting circuit 1 including said starting circuit includingsaid starting switch for rendering said fuel delivery means and saidignition means operative, means operative-as a result of theestablishment of combustion to close a holding circuit for saidfueldelivery means and energize said timing means in said firstdirection whereby said timer member is moved to said second positionresulting in the opening of said starting switch and the rendering ofsaid ignition means inoperative, a second switch closed by said timermember in said sec- 0nd position, and circuit means operative upon theopening of said holding circuit to "energize member from said second tosaid first position,

, said timing means being inoperative to move said member when it isenergized in both'or neither directions, a safety member moved by saidtiming means when it is energized in both directions, as starting switchclosed by said timer member when it is in its first position, a relaywhich, when energized, renders said fuel delivery means operative, astarting circuit for said relay including said starting switch, circuitcontrol means closing said starting circuit in response to a demand forburner operation, means operating in response to the energization oi.said relay to'energize said timing means in both directions, and meansoperated by said safety member for rendering said fuel delivery meansinoperative after a predetermined period of time, means responsive tocombustion, providing combustion is established before said fueldelivery means has been rendered inoperative, for caus ing energizationof said timing means in the first direction only and for setting up aholding circuit for said relay, and means operating as a result of thedeenergization of said relay for energizing said timer in the seconddirection.

9. A fluid fuel burner control system comprising in combination, meansfor delivering fuel to a burner, means for igniting said fuel, a timermember, a timing means which may be energized in a first direction tomove said member from a first to a second position a second direction tomove said member from said second to said first position, means beinginoperative to move said member when it is energized in both or neitherdirections, a safety member moved by said timing means when it isenergized in both directions, a starting switch closed by said timermember when it is in its first position, a relay which, when energized,renders said fuel delivery means and said ignition means operative, astarting circuit for said relay including said starting swam circuitcontrol means closing said starting circuit in response to a demand forburner operation, means operating in response to the energization ofsaid relay to energize said timing means in both'directions, and meansoperated by said safety member for rendering said fuel delivery meansand ignition means'inoperative after a predetermined period of time,means responsive to combustion, providing combustion is establishedbefore said fuel delivery means has been rendered inoperative, forcausingenergization of said timing means in the first direction only andfor setting up a holding circuit for said relay, said timer memberopening said starting switch and rendering said ignition meansinoperative, and means operating as a result of the deenergization ofsaid relay for energizing said'timer in the second direction.

10. A fluid fuel-bumer control system comprising in combination, meansfor delivering fuel to a burner, a timer comprising a timer member, afirst thermal element which, upon being heated, drives said timer memberfrom a first to a second position, a second thermal element which, uponbeing heated, drives said timer member from said second to said firstposition, said thermal element being incapable of moving said timermember when neither or both of them are heated, a starting switch closedby said timer member when it is in its first position, a relay which,when energized, renders said fuel delivery means and may be energizedin,

said timing I causing heating of ber is moved to itssecond positionandopens said starting switch, and circuit means rendered operative as aresult of the opening of said holding circuit for heating said secondthermal element and not said first thermal element to cause saidtimermemb'er tion.

11. A fiuid fuel burner control system comprising in combination, meansfor delivering fuel to a burner, meansfor igniting said fuel, a timercomprising a timer member, a first thermal element which, upon memberfrom a first to a second position, a second thermal said sec-5 0nd tosaid first position, said thermal elements being incapable of movingsaid timer member when neither or both of them are heated, a startingswitch closed by said timer member when itis in its first position, arelay which, when energized, renders said fuel delivery means andigniting means operative todeliver fuel to the burner and ignite cuitfor saidrelay including said starting switch,

I means for closing said starting circuit in response to a demand forburner operation, means including a combustion responsive switch forsetting up a holding circuit for said relay and said first thermalelement and not said second thermal element upon the establishment ofcombustion whereby said timer member is moved to its second position andopens said starting switch andrenders said igniting means inoperative,and circuit means rendered operative as a result of the opening of saidholds ing circuit for heating said second thermal element and not saidfirst thermal elementto' cause said timer member to return to its firstposition. 112-. A fiuid fuel ing said timer member when neither or bothof them are heated, a starting switch closed by said its first position,'a'

timer member when it is in relay which, when energized, renders saidfuel delivery means operative to deliver fuel to the provided combustionis established before said safety switch is opened, for setting up aholding circuit for said relay and opening the heatincluding saidstarting to return to its firstposi being heated, drives said timerelement which, upon being heated, drives said timer member from thesame, a starting cirburner control system com prising in combination;means for delivering fuel to a burner. a timer comprising a timermemsaid timer member from a first to said second to said first position,elements being incapable of mov-" 6 ing circuit for said second thermalelement ,whereby only said first thermal element remains heated and saidtimer member moves to its second position, opening'said starting switchand the heating circuit for said first thermal element,

and means responsive to the termination of combustion at the burner fordeenergizing said relay, providing said relay has not already beendeenergized, and forheating saidsecond thermal element whereby saidtimer member returns to its first position in 'which it opens theheating circuit for said second thermal element and recloses saidstarting switch.

13. A fluid fuelburner control system cocnprising in combination, meansfor delivering fuel to a burner, 'means for igniting said fuel, a

element which, upon being heated, drives. said timer-memberfrom a firstto a second position. a second thermal element which, uponbeing heated,drives said timer member from said sectimer comprising a timer member, afirst thermal ond to said firstposition, said thermal elements beingincapable of moving :said. timer member when neither or both of them areheated, a

starting switch closed by. saidwtimer" member when it is in its ffirstposition, a relay which,

when energized, renders said fuel delivery-means 'and igniting meansoperative to deliver fuel to the burner and ignite the same, a startingcircuit for said relay including said starting switch,

"means for closing said starting circuitin response to a demand forburner operation, circuit means established as a result oftheenergisation of. said relay for simultaneously heating both'of saidthermal elements. a safety switch, means said safety switch, saidlast'named means being operative onLv when both of said thermaleleestablishment of combustion, provided combustion is establishedbefore said safety switch is opened, for setting up a holding circuitfor said relayxand'openingthe circuit for said second.thermal elementwhereby onlysaid first thermalelementxremainsz heated and said timer toits 'seoondposition, rendering .said igniting means inoperative-,openingsaid the heating circuit for said first'thor'mal element, and-meansresponsive to the termination of combustion at the burner for d saidrelay, providing said r l-Y h not alreadybeen deenergined, and forvheating saidseccnd, thermal element whereby said timer memberireturnsxto its first-position in which it heatln circuit for saidsecondthermal element and recloses said starting switch.

1 4. Athermal relay unit comprising a pivoted arm, a'pair-of thermallyexpansible elements'conat one end tosaidarm on opposite sides of itspivot, a member connecting the free endsof said thermal elements,,andmeans for selectively heating sa'idthermal elements, diflerentially tocause rotation of said arm, or to the same extent,

to obtain movement of said member.

15. A combination thermal timer and safety switch actuator comprising incombination, a ro- .ucomingsaid resisting means to actuate said safetyoperated by said thermal elements forv opening :ments heated, meansresponsive to the assmsor tatably mounted mun actuating arm. i. amthermal element mounted atone end of said am,

a second thermal element mounted at the other end of said arm, a-safetyswitch. an actuating member for said safety switch mechanicallyconnected to the opposite ends of said thermal elements, said rotatablymounted switch actuatingarm assuminga mid-position when said thermalelements are at the same temperature, a snap action switch mechanicallyactuated by said rotatably mounted switch actuating. arm, said roswitchwhen said thermal elements are heated 16. A thermal relay unit,-comprising, a pair of thermal elements, first control means connected atits opposite ends to said thermal elements for actuation thereby,saidmeans assuming a midposition when said thermal elements are the sametemperature ,and'movable to either side of midposition upondifi'erentialheating of either'oi said thermal elements, and'a switchoperated by said first control means, control means connectedto theopposite ends'gof, said thermal elements, said secondcontrol means beingmovable in a longitudinal direction upon equal and nibstaotial heatingof both'of said thermal, elements, and a second switch operable by saidsecond control means.

17. A fiuid fuel burner control apparatus for controlling the operationof a 'i'uel delivery means comprising in combinations. device includingtiming means," a starting switch, means adapted for connecting saidtiming means and saidsta'rting switch in circuit relationship with a idevice which is responsive to a demand ior heat 'to mamas astartingcircuit including use we ing switch for rendering said fueldelivery means operative, said combination being adapted to close saidcircuit in resp nse to a demand by said heat responsive device, means,including. said timing means, operative as'a result of the closure ofsaid starting circuit for setting up a V the operation of said fueldelivery means, and opening said starting switch, said combination Yopted to open said holding circuit to render aid fuel delivery meansinoperative when the demand by said heat responsive device is satisfied,and means operative as a result of the deenergization of said holdingcircuit for energizing said timing means in amanner tocause it toreclose said starting switch;

SIEGll'RIED G. ISSERSTEDT.

holding circuit for

